Functions of Drilling Fluids Control Subsurface Pressure The drilling fluid creates hydrostatic pressure and this saves the borehole of the well from cave-in An overbalanced condition occurs when the drilling ﬂuid exerts(оказывает)a higher pressure than the formation pressure An underbalanced condition occurs when the drilling ﬂuid exerts a lower pressure than the formation pressure Functions of Drilling Fluids Cuttings transport The drilling ﬂuid should be able to remove rock cuttings from beneath the drilling bit, transport them up the wellbore through drillstring annulus The density and viscosity of the drilling ﬂuid are the properties that control the process Functions of Drilling Fluids Fluid-Loss Control The drilling mud should create a thin, low-permeable ﬁlter cake to minimize liquid loss into permeable formations. However, the filter cake should be able to filter through itself some portions of the drilling mud to make the drilling easier. Poor ﬂuid-loss control can cause: Surge - an increase in wellbore pressure under a bit from running into the hole or Swab - a decrease in wellbore pressure under a bit from running out of the hole Filter cake Functions of Drilling Fluids Cooling and lubricating the Rotating Bit and Drillstring The drilling mud cools and lubricates the drill string and the bit during the drilling The ﬂuid should have the ability to absorb the heat generated by the friction between metallic surfaces and formation Functions of Drilling Fluids Ensure Maximum Logging Information The drilling fluid has a profound impact on the electrical and acoustical properties of a rock. Because these properties are what logging tools measure, it is imperative that the correct selection of wireline logging tool or loggingwhile-drilling (LWD) tool for a given drilling fluid be made. Functions of Drilling Fluids Transmit Hydraulic Horsepower to the Rotating Bit The hydraulic force is transmitted to the rotating bit when the fluid is ejected through the bit nozzles at a very high velocity. In directional-drilling operations, the hydraulic force powers the downhole hydraulic motor and turns electricpower generators (turbines) for measurementwhiledrilling (MWD) and LWD drillstring equipment.